1. Field of the Invention
The present invention relates to a heater useful for an image heating device mounted on an image forming apparatus such as an electrophotographic copier or an electrophotographic printer, and an image heating device mounting the heater.
2. Description of the Related Art
An image heating device mounted on a copier or a printer includes an endless belt, a ceramic heater which contacts the inner surface of the endless belt, and a pressure roller which forms a fixing nip portion with the ceramic heater via the endless belt. If small size paper is continuously printed by an image forming apparatus which is mounted with such an image heating device, the temperature of a non-paper-passing portion in the longitudinal direction of the fixing nip portion gradually increases (temperature rise at non-sheet-passing portion). If the temperature of the non-sheet-passing portion becomes too high, it may cause damage to the components of the apparatus. Further, if large size paper is printed in a state where the temperature at the non-sheet-passing portion is high, high temperature offset of toner may occur at the area corresponding to the non-sheet-passing portion of small size paper.
As one method for preventing such temperature rise at the non-sheet-passing portion, Japanese Patent Application Laid-Open No. 2011-151003 discusses a method which uses two conductive elements and a heat generating resistor formed by a material having a positive temperature characteristic of resistance. The heat generating resistor is mounted on a ceramic substrate and the two conductive elements are arranged at both ends of the substrate in the widthwise direction of the substrate so that the current passes the heat generating resistor in the widthwise direction of the heater. The widthwise direction of the heater is the conveying direction of the paper. This flow of current is hereinafter referred to as power feeding in the paper conveying direction. The resistance of the heat generating resistor at the non-sheet-passing portion increases when the temperature of the non-sheet-passing portion increases. Thus, the heat generation at the non-sheet-passing portion can be decreased by reducing the electric current that passes through the heat generating resistor at the non-sheet-passing portion. The resistance of a device having the positive temperature characteristic of resistance increases when the temperature increases. Such characteristic is hereinafter referred to as positive temperature coefficient (PTC).
However, even if a heater configured as described above is used, the electric current flows through the heat generating resistor positioned at the non-sheet-passing portion and heat is generated.